(a) Field of the Invention
The present invention relates to a non-magnetic monocomponent color toner offering superior image density and printing efficiency because of a narrow charge distribution and good chargeability, and having superior long-term stability because of significantly improved charge maintenance, and a preparation method thereof.
(b) Description of the Related Art
Recently, demand for color toner is increasing in the field of electrophotography. Color toner is prepared by kneading and crushing, suspension polymerization, emulsion polymerization, etc. Among them, the kneading and crushing method is mainly used in terms of stability, productivity, and so forth.
In the kneading and crushing method, a binder resin, a colorant, a charge controller, a releasing agent, etc., are melted and kneaded to obtain a mixture. The mixture is cooled and crushed to a desired particle size and classified to obtain a toner. The toner is developed by frictional charging to a positive or negative charge depending on the polarity of the developed electrostatic latent image. Recently, printers adopting electrophotography, in which a laser beam is used as light source, have been leading the market. The demand for compactness, lightness, reliability, and full color is increasing rapidly. Thus, electrophotographic devices having a simple structure and offering high good quality and durability are required. Also, a toner having good printing efficiency and stable developing properties in the long term is required.
In order to satisfy the recent need for higher resolution and better image quality, the particle size of toner is becoming smaller. As the particle size of the toner decreases, the surface area per unit weight of the toner particle increases. As a result, the surface characteristics affect charging and particle characteristics of the toner. As the particle size becomes smaller, the charging characteristic is more affected by the charge control agent. In general, a metal complex, a chromium-containing metal dye, or a quaternary ammonium salt is used for negative charging, and nigrosine or a quaternary ammonium salt is used for positive charging, as the charge control agent. The charge control agent is melted and kneaded along with a binder resin, a wax, a colorant, etc., and crushing and classifying are performed to obtain a toner.
The raw material of the charge control agent may have quite a broad particle size distribution. Although the charge control agent particles may be broken down during melting or kneading, the original particle size determines the characteristics of the charge control agent. If the charge control agent has too large a particle size, the binding ability with the binder resin decreases, so it tends to be separated from the toner during crushing. As a result, many toner particles do not contain the charge control agent and the charge distribution becomes broader, so background contamination or fogging tends to occur. Otherwise, if the charge control agent has too small a particle size, most of the charge control agent particles exist inside the toner, so they do not contribute to improvement in charging characteristics.
Accordingly, it is required to improve binding ability with the binder resin, charge distribution, and charge maintenance by specifying the particle size and distribution of the charge control agent.